Modified lanthanide-doped carbon dots as a novel nanochemosensor for efficient detection of water in toluene and its potential application in lubricant base oils

A fast and efficient method was developed for obtaining europium(III)-doped surface-modified carbon dots with a hydrophobic coating. This surface functionalization improved the dispersibility of the nanoparticles in non-polar media, as well as modified the accessibility of water molecules to the europium ions. These two features allowed studying the application of doped carbon dots as moisture nanochemosensor, demonstrating high stability over time of both the photoluminescent signal intensity and the stability of the dispersions. The developed nanochemosensor was used to determine water in toluene with a detection limit of 8.5 × 10−4 M and a quantification limit of 2.4 × 10−3 M. The proposed system matches and even improves other methodologies for water determination in organic solvents; it has a low detection limit and a fast response time (almost instantaneous) and requires neither expensive material nor trained personnel. The results suggest a promising future for the development of a new sensing phase for moisture determination in lubricant base oil. Graphical Abstract Supplementary Information The online version contains supplementary material available at 10.1007/s00604-023-05659-5.


Synthesis of europium-doped carbon dots (EuCDs)
1 g citric acid plus 0.5 g glutathione and 0.2 g of EuCl3·xH2O were solved into 10 mL ultrapure water and ultra-sonicated until obtaining a homogeneous mixture. The final solution was poured into a porcelain crucible and introduced into an oven at 180 °C for 150 minutes. The mixture in the crucibles was hydrated every 30 minutes (4 times) with ultrapure water in order to control the carbonization of the reagents. After completing the reaction, the crucibles were removed from the oven and let to cool down in a desiccator, yielding a brownish-orange solid which was dispersed into 15 mL ultrapure water using an ultrasound bath. These final suspensions were purified by dialysis versus water using membranes MWCO 1kDa. Then, the solvent was partially removed using a rotatory evaporator and thereafter lyophilized, obtaining a brown-greenish powder.

Hydrophobic modification of EuCDs surface
0.5 g EuCDs were dispersed into 30 mL water and its pH was raised over 10 with NaOH.
Meanwhile, 0.3 g (~0.75 mmol) MTOA was dissolved into 30 mL toluene. Both phases were then mixed and vigorously shaken for 72 hours. The organic phase was recovered and washed two times with 30 mL NaOH solution (pH 9 -10). Toluene was removed with a rotatory evaporator until dry. A brownish viscous almost-solid paste-like material was obtained, EuCD-MTOA.

XPS Analysis
According to the work by Gengenbach et al. [22], identifying the organic functionalities in a high resolution C1s spectrum by curve-fitting and assignation of binding energies is poorly trustable and almost speculative when having more heteroatoms than oxygen, as it is our case. Furthermore, the combination of graphitic and non-graphitic carbon which is present in our samples requires different curve profiles and makes the interpretation uncertain. Therefore, we relayed the interpretation on the comparison of EuCD and EuCD-MTOA spectra. Such high binding energies are related to highly oxidized carbon states and are sometimes assigned to C=N [34] or to C=O [35], but such assignation is mainly speculative in our case according to Gengenbach's discussion [22]. Nevertheless, we can assume that this band is due to functional groups present in EuCD surface but not in EuCD-MTOA surface, which is consistent with the fact that EuCD-MTOA surface is coated with MTOA (mostly C-C and C-H bonds), which hinders the detection of the underlying functionalities. Similarly, an intense europium peak can be detected for the EuCD sample at 1133.4 eV (Figure SI.2 right), belonging to Eu3d5/2 which fits the position with that of Eu3d5/2 for the inorganic salt EuCl3. This peak is despicable and almost inexistent in the case of EuCD-MTOA, again because europium is somehow 'hidden' below the organic coating of MTOA.

Morphologic characterization
EuCD-MTOA were morphologically characterized using HRTEM and STEM. The  (63Eu) is revealed by a more intense signal than that of the rest of the atoms of the matrix, as the latter show very low atomic numbers and, therefore, a weaker signal [37]. This result was corroborated by semiquantitative EDX analysis which reflected a 0.16 ± 0.04 % atomic percentage of europium.

Protocol for determination of water in oil
The proof of concept for moisture determination in lubricating oils was performed as follows: First, a suspension of the EuCD-MTOA was prepared in anhydrous toluene. An appropriate volume of this suspension was added to each flask and dried in a vacuum oven (final concentration of EuCD-MTOA: 0.05 % w/v). After this step, a proper aliquot of the lubricant oil was added to the flask in order to reach a concentration of 10 % v/v.
Next, appropriate volumes of anhydrous toluene and water-saturated toluene were added to perform the calibration curve, starting (zero curve point) with anhydrous toluene up to a final concentration of water given by the water-saturated toluene. The solutions were homogenized gently and the whole procedure was carried out quickly to avoid any contamination by environmental humidity.

Method features compared to other published systems
The determination of water in organic solvents can be faced from different perspectives.
The use of a naphtalimide-based molecule attached to a carbon dot [38] provides a very sensitive method for determining water in toluene, which is also applicable in lubricant oils, even those with high viscosity. However, that methodology requires a tedious synthesis procedure. Although less sensitive, our proposal uses a fast and simple synthesis which, additionally, produces few residues and is therefore 'greener'.
Similarly, the use of Cs4PbBr6 nanocrystals provides a ratiometric fluorescent method for the determination of water in some organic solvents with a similar detection limit to ours and applicability in a wider range of solvents [39]. Nevertheless, since the analytical features are similar, the use of less toxic Carbon Dots should be rather preferred to materials containing lead or toxic metals.
Likewise, there are published descriptions of carbon dots used for the determination of water content in different solvents [40]. For example, carbon dots from 2,5-dihydroxyterephthalic acid can be used for water determination in ethanol and other solvents using a fluorescence ratiometric approach. However, this methodology has a worse limit of detection than the one presented here, as well as a longer response time.